One of the techniques for separating gas mixtures having similar molecular masses is of course gas diffusion. In this technique the gas mixture is circulated under pressure inside tubes formed by a microporous wall. In a known technique the tubes are formed on the one hand by a macroporous ceramic tube, generally known as a support, which is lined with a microporous layer deposited on such inner wall. The assembly formed by the tube or "support" itself and the microporous layer enables the total porosity of the coated tube or "barrier" to be adapted so as to obtain an optimum coefficient of gas separation. This technique is more particularly used for the separation of two gases corresponding to different isotopes of the same single body.
However, it has been found that when gas flows inside the barriers or tubes, one of the phases accumulates to some extent on the inner surface of the tube. This is why according to the invention the inner face of the tubes is formed with reliefs to set up considerable turbulences in the gas inside the tubular elements, so as to prevent such an accumulation of one of the phases and thus to enhance the coefficient of separation by gas diffusion.
The invention relates exclusively to the making of the support--i.e., the macroporous tube. In the most conventional form, such cylindrical tubes are made by the usual ceramic methods--i.e., by the mixing of ceramic oxides, thermally eliminable organic binders and a liquid phase (generally water). After these main elements have been mixed, the mixture takes the form of a plastic, deformable paste which is turned into a tube by means of a piston press or a screw extender. The paste is introduced into the press cylinder and forced under pressure through an annular space bounded by an outer nozzle and an inner punch, which define the external and internal diameters of the tube to be produced.
When cut to the required length, the tube is then placed on rollers which rotate and advance in a drier whose temperature enables the liquid phase to be eliminated and results in a non-deformable, robust tube which can be handled or automatically conveyed to the other installations for performing the subsequent operations of preliminary firing (elimination of organic binders) and actual firing at elevated temperature, which gives the tube the required properties (for example: permeability, dimensions of pores) and a high mechanical strength, the tube being then if necessary conveyed to other installations for transforming the material.
On emerging from the molding machines (a screw extruder or the like) the element takes the form of a tube which is plastically deformable but relatively resistant to elongation and tearing.
The method of making reliefs or imprints on the inner face of the tube is carried out at this stage in tube processing.